Magnetic resonance imaging (MRI) is a clinically significant diagnostic tool because of its ability to readily image and distinguish different types of soft tissue. Water protons predominantly found in soft tissue have a detectable, albeit weak, nuclear magnetic moment or spin. The nuclear magnetic moments are randomly oriented, but become aligned when placed in a strong static magnetic field (B0). However, the nuclear magnetic moments are too weak to be measured in such a static magnetic field.
By applying an oscillating magnetic field (B1), the nuclear magnetic moments may be made to oscillate or precess at a frequency which is characteristic of the soft tissue. The oscillating magnetic field preferably comprises a rotating radiofrequency (RF) magnetic field, which also creates a fictitious field aligned with the static field. By tuning the RF magnetic field (B1), the static magnetic field (B0) may be effectively cancelled out.
The oscillating nuclear magnetic moments create a magnetic flux that varies as a function of time, which induces electric current flow in a conductive coil, and therefore may be detected using a loop antenna. The electric signal induced by the time varying magnetic flux in the loop antenna is proportional to the nuclear magnetic moments. Preferably, the oscillating magnetic field (B1) is applied such that the nuclear magnetic moments oscillate in a plane perpendicular the static field (B0) where the received signal may be maximized.
Because water protons oscillate at the same resonant frequency, the received signal may be used to detect water in a sample. In MRI, the magnetic filed is spatially varied using gradient magnetic fields such that the amount of water in a given region may be determined. Because water protons are predominant in soft tissue, this permits localization of soft tissue. Different types of soft tissue may be further identified using various contrast techniques which utilize relaxation time, chemical resonant frequency, or other attributes to distinguish different types of soft tissue.
In a typical MRI scanner, the RF magnetic field (B1) is generated by an RF coil and the gradient magnetic fields are generated by three gradient coil windings (X, Y, Z) which surround the RF coil. To prevent the RF magnetic field (B1) from interfering with the surrounding gradient coils windings, it is desirable to restrict the spatial extent of the RF magnetic field (B1) using an RF shield around the RF coil. The most straightforward approach for such an RF shield is to use a thin cylinder of copper foil around the RF coil. The RF shield is kept as thin as possible because the switching of field gradients during the imaging process induces eddy currents in the RF shield that may corrupt the gradient waveforms.
A more effective alternative is to cut up the RF shield into a number of smaller areas to decrease the eddy currents, but to reconnect the pieces with multiple RF bypass capacitors. The capacitors are chosen so that the shield appears to be a short circuit at radio frequencies, but conducts minimal currents at the audio frequencies produced by the field gradient switching.
An example of this alternative approach is disclosed in U.S. Pat. No. 4,642,569 to Hayes et al., which describes a shield fabricated from thin PTFE circuit board double sided with copper foil, wherein the copper foil is cut into discrete regions to form an overlapping array of conductive regions on either side of the circuit board. This arrangement provides an intrinsic capacitance between the two sides of the circuit board which serve as bypass capacitors across the cuts in the foil surfaces. The cut patterns in the circuit board are chosen to run largely parallel to the expected RF currents in the shield, so a minimal number of capacitive gaps are encountered as the RF current completes its path around the shield. The capacitive elements are selected to provide a short circuit at radio frequencies.
RF shields can also be used to redirect or reflect the distribution of magnetic fields generated by an RF magnetic coil. For example, in a head coil such as a birdcage resonator, placing a shield or endcap across one end of the birdcage resonator will improve the field homogeneity at that end by preventing the RF flux from diverging out. An example of an endcap shield is disclosed in U.S. Pat. No. 4,692,705 to Hayes, which describes a highly conductive RF endcap shield to prevent bulging of the RF magnetic filed.
The highly conductive RF endcap shield prevents bulging by virtue of electromagnetic boundary conditions. A first boundary condition requires the perpendicular component of magnetic field to be zero at the surface. A second boundary condition requires that the tangential component of the electric field be zero at the surface. The second boundary condition is not always desirable because of possible de-tuning effects, dielectric losses, and the need for larger electric fields elsewhere.